TITLE: GCN CIRCULAR NUMBER: 11326 SUBJECT: GRB101008A, GROND detection of a fading afterglow DATE: 10/10/09 10:58:06 GMT FROM: Marco Nardini at MPE M. Nardini, P. Schady, J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 101008A (Swift trigger 435903; Baumgartner et al., GCN#11318) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory (Chile). Observations started at 00:26 UT 2010 October 9th, 7.8h time after the GRB trigger, and lasted 2 hours. They were performed at an average seeing of 1.2 and at an average airmass of 2.5. In all our g'r'i'z'JHK images we detect the 3 objects reported by Moskvitin et al., GCN#11324. While sources S1 and S3 do not show any clear fading with respect to that reported by Moskvitin et al., GCN#11324, our preliminary photometry shows that S2 has faded with respect to the value reported by Moskvitin et al. (GCN#11324). Calibrating our images against the same USNO star as Moskvitin et al., we calculate a fading of ~0.7mag between the GROND preliminary photometry centred at 8.7h after the trigger and that reported in Moskvitin et al. We therefore conclude that this is the optical/NIR afterglow of GRB101008A. Our photometry is not corrected for foreground extinction of E(B-V)=0.19. The detection in g' band implies a redshift constraint z<3.5.